Distributed Raman amplifiers (DRA) in optical telecommunications networks use transmission fibres to provide optical gain to signal channels. The distributed nature of the amplifier hinders the control of the amplifier, including the gain and safety requirements. This is because the amplifier controller does not know the characteristics of the amplification medium (i.e. the transmission fibre) with which it is to operate. This is unlike a discrete amplifier, such as an Erbium (Er) Doped Fibre Amplifier (EDFA) or discrete Raman amplifier.
To improve the situation, it is valuable for the DRA to know certain parameters of the optical transmission fibre span with which it is to operate. The parameters may include fibre type, fibre loss, fibre length, fibre gain coefficient and chromatic dispersion (CD), which can be used either within an optical network controller or used to determine the fibre type in the span. This information is used to control the operating characteristics of the DRA, as well as the safety system.
U.S. Pat. No. 6,850,360 describes a Raman amplifier system with diagnostic capabilities. Backscattered Raman pump light is used in optical time domain reflectometry (OTDR). This is very valuable and will provide information on the span losses and Raman gain coefficient, but the technique disclosed in U.S. Pat. No. 6,850,360 does not measure other parameters of the optical fibre.
Several methods exist to measure fibre CD. One conventional method, known as phase shift, uses two modulated lasers specifically designed for CD measurement and operating at different wavelengths. Light from the lasers is propagated through the optical fibre and the phase difference in the received laser light provides a measure of the delay experienced by the light from each laser, which is used to determine the CD of the fibre. To get an accurate measurement of CD against wavelength one laser is fixed and the other laser is tuned across a defined wavelength range to get a wavelength dependence of phase delay and thus the CD of the fibre.
In order to determine the parameters of the optical fibre, it is necessary to have test equipment and/or trained personnel at each end of an optical fibre to be tested. Optical fibre spans may have lengths in excess of 100s of kilometers, which places a significant burden on resources when determining parameters of an optical fibre.